Digital communications show a significant development with mobile telephony, new digital television standards, and satellite transmissions. The significant progress in that domain and the great diversity of operators have made information a “good” as easy to exchange as goods can be.
That great diversity of operators in the transmission chain has introduced the need, inside the decoder of the digital communication system, to detect, in addition to the classical parameters of time and phase synchronization, a supplementary parameter which is that of spectral inversion.
As known and illustrated in FIG. 1a, at the time of a demodulation, a receiver performs the mixing of the spectrum of the received signal with a carrier frequency (OL). Depending on whether that carrier is a frequency is below or above the spectrum of the frequency to be processed, an inversion of the spectrum can occur. FIG. 1a illustrates a typical situation where no spectral inversion is introduced.
FIG. 1b illustrates on the opposite a situation of FIG. 1a, wherein an inversion has occurred within the frequency spectrum. That inversion is not difficult to manage, especially when the spectrum of the signal to transmit is perfectly symmetric as is the case for an amplitude modulation.
On the other hand, when the modulation is more complex, a spectral inversion is a problem because data may not be detected. Using a phase locked loop mechanism, the detector is properly being locked on the carrier frequency. However, this does not permit data reception when a spectral inversion occurs.
The problem of spectral inversion needs to be corrected because, in today's digital telecommunications, multiple modulations/demodulations often times affect the signal transmission and reception, especially when various operators are involved on a same signal.
With this brief background, there are two possibilities to correct the spectral inversion: either by means of an inversion of the signal in quadrature, or by inverting both real and quadrature signals after a rotation of π/2.
One classical method to correct spectral inversion is with a software inversion of the channels in phase along with squaring of the demodulated signal. To this end, the classical receiver “presumes” during the processing of a received signal that this signal is not affected by an unspecified spectral inversion. If the detection fails while synchronized on the carrier, the receiver, after a certain period, concludes that a spectral inversion might have occurred and thus introduces a channel inversion in phase with squaring to re-establish the original spectrum.
This mechanism, which is universally used, is satisfactory up to a certain point, but has the disadvantage of introducing a blocking latency time in the processing of the demodulated signal for most modern applications.
Accordingly, what is needed is a method and system to to overcome the problems with the prior art spectral inversion by allowing a quasi-immediate detection of a spectral inversion in a received signal so as to guarantee the instantaneous processing.